DS has crawled over tiles, carpet, grass etc etc and has never had a need for knee pads. Although when i used to try and get on the floor and crawl around with ds i was in so much pain. Their little knees are made for crawling! Definately a clever marketing ploy.
They adapt too I find, like do a crab crawl or a hands and feet crawl. I used to get sore knees too lol but the thing is.. they DON'T have kneecaps to get sore at their age, it's cartlidge so it's soft. I wish I had thought of the marketing ploy though rofl!!! I never used anything either.